The present invention relates to a system for communication between two communication platforms and a method for communication between two communication platforms. In particular, the present invention relates to a communication between a near-earth returning platform (ezP) and a remote platform (eS). In addition, the present invention relates to a system and a method for communication between these two communication platforms using a bidirectional asymmetrical communication link.
Various communication methods are currently known for establishing and maintaining a communication link between communication platforms. In particular mobile communication platforms, for example, communication platforms embodied as vehicles, such as aircraft and satellites, use wireless communication links largely due to the relative mobility of the communication platforms. Such communication links may use electromagnetic waves, for example, and may be embodied as radio communication links or optical transmission methods, for example, using laser light of a defined wavelength.
There are known methods for communication between satellites in the same orbits (LCTSX) and/or in different orbits (EDRS, SPOT4 ARTEMIS), between satellites and manned aircraft (ARTEMIS FALCON), unmanned aerial vehicles or UAVs (LCT UAV) and between a satellite and a ground station on the earth's surface (ARTEMIS OGS, LCTSX OGS).
Corresponding communication methods enable a transfer of information between the individual communication platforms. These known communication methods have in common the fact that the methods and/or the communication links they use are designed to be symmetrical as such. In other words, the methods used are identical in the principles used, in particular with respect to the type of modulation and/or the data rate implemented in a first link and/or a first partial communication connection, for example, an uplink between a near-earth platform and a satellite as well as, in a second link or a second partial communication connection, a downlink between a satellite and a near-earth platform, for example.
It should be noted, however, that the communication channel between two such platforms, for example, between a near-earth platform and a satellite, is not itself designed to be symmetrical. To this extent, it may appear advantageous to adjust the communication link between two platforms to the possible asymmetry of a communication channel to a greater extent.
Thus, exemplary embodiments of the present invention provide a communication link adapted to the specific requirements of communication between two communication platforms. This may be, for example, a laser communication link between a satellite and a near-earth platform (e.g., a returning platform). The satellite or the remote platform may be a platform for operation outside of the earth's atmosphere, in particular in a Low Earth Orbit (LEO) or a geostationary orbit (GEO), while the near-earth platform, in particular a returning platform, is a platform for operation in the earth's atmosphere or at the border thereof, in particular an aircraft, more specifically an airplane. The use of additional aircraft such as airborne vehicles resembling balloons or zeppelins or the like is also conceivable.
It is particularly advantageous to use different types of signal modulation that permits an effective and broadband bidirectional communication link between a remote platform and a near-earth returning platform. To do so, different types of communication may be adapted to the asymmetry of the transmission channel between the remote platform and the near-earth returning platform.
When a different type of communication is mentioned within the context of the present invention, this should not be understood to mean an exclusive differentiation into a possible and/or implemented data rate that instead should be based on a difference in the technical implementation of each partial communication link, for example, using different types of signal modulation and/or transmission methods, although these may ultimately lead to different data rates as a result.
Thus, within the scope of the present invention, an asymmetrical communication link is based on a different type of communication and/or on a different communication method for each partial communication link, in particular not being based on a different data rate of the respective partial communication link with the same type of communication and/or with the same communication method.
The communication link between two communication platforms should thus be implemented as a bidirectional asymmetrical communication link by using different types of communication and/or transmission methods for a first partial communication link and a second partial communication link.
According to the present invention, special aspects of laser communication between a satellite and a near-earth returning platform in this context are related to a near-earth returning platform with special consideration of the special features of laser communication through the earth's atmosphere and/or between the earth's atmosphere and outer space, in particular taking into account the costs and efficiency for a laser communication terminal (LCT) installed in satellites in outer space as well as the corresponding costs and complexity for a laser communication terminal.
According a system for communication between two communication platforms, a method for communication between a near-earth returning platform and a remote platform as well as the near-earth returning platform itself and a remote platform are disclosed.
According to one exemplary embodiment of the present invention, a system for communication between two communication platforms is provided, having a near-earth returning platform (ezP) and a remote platform (eS), such that a communication link is established between these two platforms, the communication link being embodied as a bidirectional asymmetrical communication link.
According to another exemplary embodiment of the present invention, a method for communication between a near-earth returning platform and a remote platform is provided, comprising communication between the near-earth returning platform and the remote platform using a bidirectional asymmetrical communication link.
According to another exemplary embodiment of the present invention, a near-earth returning platform is provided, which is designed for communication with a remote platform using a bidirectional asymmetrical communication link.
According to another exemplary embodiment of the present invention, a remote platform is provided, which is designed for communication with a near-earth returning platform using a bidirectional asymmetrical communication link.
According to one aspect of the present invention a laser communication terminal (LCT) is installed on both the remote satellite platform and the near-earth returning platform. The laser communication terminal of the remote platform may have at least the particularly efficient reception method of heterodyne reception as well as having at least the transmission method of amplitude modulation. Furthermore, the laser communication terminal of the near-earth returning platform may supply at least the transmission method of heterodyne reception as well as at least supplying the reception method of amplitude modulation.
In this context heterodyne reception may be understood to refer to optical heterodyne reception as well as homodyne optical heterodyne transmission with phase modulation.
The communication method according to the present invention, amplitude modulation reception of the near-earth returning platform and heterodyne reception of the remote platform offers advantages of ideally designing and using the respective transmission units of the other platform, based on the communication channel between the near-earth returning platform and the remote platform.
The emission of a laser beam of the transmission unit of the near-earth returning platform in the direction of the remote platform is first disturbed by the atmosphere because it is assumed that the near-earth returning platform operates inside the earth's atmosphere. However, outside of the earth's atmosphere, i.e., above the Kármán line, the wavefront of the laser beam is actually improved due to the further propagation of the laser beam in outer space, where there is no air, because atmospheric interference in general is local and on a small scale, and therefore such interference is deflected out of the laser beam in further propagation until it is received at the remote platform. The wavefront received by the remote satellite platform is thus essentially free of interference and is therefore suitable for coherent reception.
In the opposite case, i.e., transmission from the remote platform to the near-earth returning platform, the laser beam of the remote platform initially undergoes interference-free propagation through outer space but is possibly subject to interference on the last portion of its path, i.e., the last kilometers through the earth's atmosphere. Reception based on amplitude modulation, however, is insensitive to phase front interference, such as that caused by the earth's atmosphere, for example, and may thus be implemented even with reception lenses of a lower quality and thus of a lower cost as well.
The communication method according to the present invention thus uses heterodyne reception in the uplink between a near-earth returning platform and a remote platform, in particular phase modulation, while using amplitude modulation in the downlink between the remote platform and the near-earth returning platform.
The transmission method is thus asymmetrical with respect to uplink and downlink in use of the respective transmission method. At the same time this results in a lower data rate in downlink than in uplink by a factor of essentially 10 because a higher data rate, e.g., in the range of 2 to 6 Gbit/s in particular 2.8 Gbit/s can be achieved when using phase modulation in the uplink in comparison with a data rate of 100 to 200 Mbit/s in downlink. At the same time the transmission power used can be adjusted in accordance with the respective platform. Thus, the remote platform, for example, may use a power in the range of 1 to 5 W, for example, 1.5 W for communication while the near-earth returning platform may transmit in the range of 20 to 30 W.
The present invention is thus particularly, but not exclusively, suitable for information collecting platforms of the near-earth returning platform that may be forwarded to another station by using the remote platform as a relay station. The additional station here may be a satellite ground station, for example, or another remote platform, i.e., another satellite which also operates as a relay station and thus increases the possible range of the data transmission, for example, by a downlink there to a ground station, which could not be reached by the actual remote platform connected to the near-earth returning platform.
The near-earth returning platform may be, for example, an unmanned aerial vehicle (UAV), which in turn collects information, e.g., image information about the earth's surface, by means of various detectors and/or camera systems. The volumes of data thereby generated, possibly enormous volumes of data, are sent to the remote platform for further distribution and/or forwarding by uplink, which is fast in comparison with downlink. This connection may be implemented here essentially permanently online or also with interruptions.
The near-earth returning platform may thus in turn store the collected information temporarily and relay it only on an individual case basis, for example, when a communication link is established with the remote platform.
One possible scenario here might be that a plurality of near-earth returning platforms operates in a similar region and the respective information collected is to be forwarded at certain points in time to the remote platform. Thus, a plurality of near-earth returning platforms can be operated from one remote platform. Necessary repositioning of at least the laser terminal on the remote platform in relation to a certain UAV may be assumed to be a known process in this context and will not be explained further here.
Communication devices on the remote platform, for example, a laser communication terminal on a satellite may be especially optimized with respect to power consumption, sensitivity of a receiver, weight and space required, for example, due to the given factors of use in outer space. The development and use of highly efficient but complex reception units are necessitated by the expected long operating times without maintenance, for example, 8 years or 15 years for low-earth orbit platforms or geostationary platforms.
Optical heterodyne reception, in particular homodyne optical heterodyne reception with phase modulation may be regarded here as one of the most sensitive and efficient reception methods. In other words, in heterodyne reception the smallest number of received photons may allow detection of a received bit. For example, for a heterodyne reception element, nine photons may be suspicious per bit at a bit error rate of 10−9. Other optical communication methods may require a greater number of photons per bit by a factor of at least 2, which would necessitate a larger reception aperture, which at the same time would also increase the weight of a laser communication terminal at the remote platform.
Optical heterodyne reception may be assumed to be especially free of interference because it is not subject to interference due to sunlight or due to controlled exposure to laser light that does not correspond exactly to the frequency and direction of the transmission unit of the additional laser communication terminal. Reception by one laser communication terminal located on a near-earth returning platform and/or the information thereby transmitted in the uplink to the laser communication terminal of the remote platform may also be ensured in possible interference measures through the use of the type of communication and/or the communication method of homodyne transmission.
A laser communication terminal of a remote platform that has the transmission methods of amplitude modulation communication as well as heterodyne communication may also be capable of communicating with a near-earth returning platform as well as with additional remote platforms, for example, an earth observation satellite or even more remote relay stations at a high data rate by means of coherent transmission and reception methods while at the same time maintaining a near-earth returning platform adapted to its specific requirements, for example, being smaller, lighter, serviceable and less expensive.
A laser communication terminal on a near-earth returning platform may require fundamentally different boundary conditions than a laser communication terminal on a remote platform. For example, it may be necessary to keep the cost of a near-earth returning terminal much lower than the cost of a laser communication terminal on a remote platform, because the platform costs per se are lower on the whole. In addition, such a system need not be designed for eight to fifteen years of maintenance-free operation through regular maintenance intervals. Thus, a relatively simple receiving segment or laser communication terminal may preferably be used, its efficiency is of secondary importance, because in general sufficient power and cooling capacity are available on a near-earth, returning platform.
In addition, it may be necessary to take into account the influence of the atmosphere on laser communication. Coherent reception may be impaired by lower strata of the atmosphere if the diameter of the turbulence cells in the line of sight between the two communication platforms is smaller than the reception aperture.
The so-called Fried parameter or the atmospheric correlation link may be used to measure atmospheric interference. This corresponds to the size of turbulence cells within which the mean quadratic error of the phase interference is 1 rad2. It is generally known that a reception aperture should be smaller than the Fried parameter to enable coherent reception and/or, in the case of traditional astronomy, observation of stars without interference.
However, amplitude-modulated reception may be assumed to be insensitive to phase front interference. At the same time, the optical quality of the reception lenses may be reduced in comparison with coherent reception. This may result in a definite cost advantage for an amplitude-modulated receiver whose line of sight for reception crosses through the atmosphere.
Typically, the data transmission rate from the near-earth returning platform to a remote platform is many times greater than that from the remote platform to the near-earth returning platform. This may result from the fact that, for example, the near-earth returning platforms usually serve as local information collecting sources, for example, UAVs over crisis regions, and transmit their collected data to remote platforms that assume the function of a data relay node.
In addition, preferred embodiments of the system according to the invention for communication between the two communication platforms are described here.
According to another preferred embodiment of the present invention, the system may have a first partial communication link to a first type of modulation between the near-earth returning platform and the remote platform and thus from the near-earth returning platform to the remote platform, and a second partial communication link with a second type of modulation between the remote platform and the near-earth returning platform and thus from the remote platform to the near-earth returning platform, such that the first type of modulation is not the same as the second type of modulation.
According to another preferred embodiment of the present invention, the first and second types of modulation may each be a type of modulation from the group consisting of amplitude modulation and phase modulation. In particular the first type of modulation may be the phase modulation type of modulation and the second type of modulation may be the amplitude modulation type of modulation.
The use of different types of modulation for each partial communication link and thus for uplink and downlink between the communication platforms makes it possible to adjust the communication link to particular physical factor sand in particular to take into account the asymmetry of the transmission channel between the platforms.
According to another preferred embodiment of the present invention, the near-earth returning platform may be designed for performing a first transmission method and a first reception method, wherein the first transmission method is a heterodyne transition method and the first reception method is an amplitude modulation reception method, wherein the remote platform is configured to perform a second transmission method and a second reception method, wherein the second transmission method is an amplitude modulation transmission method and the second reception method is the heterodyne reception method.
According to another preferred embodiment of the present invention, the near-earth returning platform may have a first transmission electronic unit and a first reception electronic unit, and the remote platform may have a second transmission electronic unit and a second reception electronic unit. The first transmission electronic unit may be equipped for transmission using the first transmission method, while the first reception electronic unit may be equipped for reception using the first reception method; the second transmission electronic unit may be equipped for transmission using the second transmission method and the second reception electronic unit may be equipped for reception using the second reception method.
The different communication methods of the heterodyne method and of the amplitude modulation method may take into account an asymmetry of the data channel and/or communication channel between the platforms and in particular may also be adapted to further factors of the respective platform, for example, the available power and/or the required maintenance intervals.
According to another preferred embodiment of the present invention, the remote platform may also be configured to perform a third transmission method wherein the third transmission method may be a heterodyne transmission method for communication with another remote platform.
It is possible in this way to likewise design a communication link, in this case a bidirectional symmetrical communication link between two remote platforms. It is thus possible to establish a relay connection, for example, between these two remote platforms to connect a remote platform that is in turn connected to the near-earth returning platform, for direct communication with a ground station at a great distance in order to relay the data collected from the near-earth returning platform to this ground station.
According to another preferred embodiment of the present invention, the communication link may be an optical communication link, in particular as a laser communication link.
Such a communication link may represent a preferred interference-resistant and/or interference non-susceptible communication link that may implement an information transmission with a comparatively low energy use.
According to another preferred embodiment of the present invention, the near-earth returning platform and the remote platform may each have a laser communication terminal, wherein the laser communication terminal has a laser element embodied as a transmission element and an optical detector element embodied as a reception element.
The detector element may be, for example, an optical reception diode, which may more preferably be adapted to a certain transmission of the optical communication link. At least one optical element, for example, a transmission and/or reception telescope, which may be embodied as a shared element or as a separate element, may be arranged on the laser element and/or on the optical detector element.
Thus, at least in the case of the near-earth returning platform, different lenses may be used for the transmission and reception light. Furthermore, a camera element may be used as an acquisition detector and/or as a tracking detector, at least on the near-earth returning platform, to implement a stable communication link between the near-earth returning platform and the remote platform by position tracking of the other platform and by adjusting the local alignment with respect to same.
According to another preferred embodiment of the present invention, at least the near-earth returning platform or the remote platform may have a plurality of laser communication terminals.
This makes it possible, for example, to implement a plurality of communication links, in particular simultaneously, with one or more platforms. With respect to the remote platform, for example, this may be in a communication link with a near-earth returning platform and with another remote platform at the same time in order to serve as a relay station, for example, and to relay data received essentially instantaneously from the near-earth returning platform to the additional remote platform, for example.
According to another preferred embodiment of the present invention the near-earth returning platform may be a platform for operation in the earth's atmosphere, in particular an aircraft, such as an airplane, and the remote platform may be a platform for operation outside of the earth's atmosphere, in particular in a low earth orbit or a geostationary orbit, such as a satellite.
In addition, preferred embodiments of the method according to the invention for communication between a near-earth returning platform and a remote platform are described here.
According to another preferred embodiment of the present invention, the method may include communicating between the near-earth returning platform and the remote platform using a first partial communication link with a first type of modulation and communicating between the remote platform and the near-earth returning platform using a second partial communication link with a second type of modulation, wherein the first type of modulation is not the same as the second type of modulation. In addition, the first and second types of modulation may each be a type of modulation from the group consisting of amplitude modulation and phase modulation; in particular the first type of modulation may be the phase modulation type of modulation and the second type of modulation may be the amplitude modulation type of modulation.